The invention relates to filters consisting of filter paper or paper-type non-woven material. Said filters partially or wholly consist of fibrous material containing cellulose.
Filter paper is a type of paper produced from cellulose, plastic fibers or glass fibers and used for the filtration in households, technical applications and for analyses. Paper-type nonwoven materials are composite materials consisting of fibrous material containing cellulose. Filter bags, for example, or filter cartridges produced by winding or folding are manufactured from filter paper or paper-like nonwoven materials. The filters so produced are disposed of after they have been used once. In connection with filters used as one-way, disposable articles it is deemed desirable that such filters are biodegradable for reasons of environmental protection. This, however, is achieved only if the filters exclusively consist of cellulose.
Filters generally serve for separating solid particles from gases or liquids. Especially in connection with the filtration of aqueous media, however, it is frequently deemed desirable that certain ions can be jointly separated as well, either in order to achieve an additional purification effect or to obtain an enrichment in the filter paper. This relates in particular to toxic heavy metals as well as to hardening constituents in drinking water, as well as to the concentration of metal traces in larger water samples for the purpose of simpler analytical detection.
No fibers for filter materials have become known heretofore that have an adequate capacity for absorbing hardening constituents in order to effect a noticeable enhancement in the production of beverages in the presence of the usual degrees of water hardness, on the one hand, and which are capable of binding heavy metals with adequate strength so as to effect a notable flavor enhancement with the possible low concentrations on the other. Commercially available systems for improving the quality of drinking water, therefore, comprise a cartridge filled with ion exchangers as their core component. However, such cartridges are known to pose problems due to the multiplication of germs if there is no flow-through in the water system, and they load the environment because such cartridges can be recycled only incompletely.
The invention was based on the problem of providing filters consisting of filter paper or paper-type nonwoven material that are characterized by a high filtration capacity and are additionally capable of absorbing and binding undesirable foreign or attendant substances in the filter medium such as, for example hardening constituents or heavy metals. Furthermore, the problem of the invention is to provide suitable methods for producing the filters.
The problem is solved according to the invention by the features specified in claim 1. The features relating to the proposed methods for producing the filters are the objects of claims 2 to 13.
The filters made of filter paper or paper-like nonwoven material partially or wholly consist of fibrous material containing cellulose. The properties of the filters are substantially enhanced by a special treatment of the material containing cellulose, either prior to or after the manufacture of the filter paper. According to the invention, the treatment is carried out in such a manner that the material containing cellulose is at least partially carbamided with urea up to a nitrogen content of 1 to 4% by mass bonded in amino-methane acid ester groups (carbamide groups), and phosphorylated with phosphoric acid or ammonium phosphate up to a phosphorus content of 3 to 8% by mass. In addition to high filtration capacity, the filter produced from cellulose-containing material so modified additionally possess the special properties of binding hardening constituents as well as toxic heavy metals, which impair the flavor. An enhanced swelling property of the cellulose fibers is obtained by such a treatment, and a broader field of application is obtained in that way for the filters, which can be preferably employed for the separation of mechanical impurities from liquids and gases. In the case of aqueous solutions that need to be filtrated, the filters possess the advantageous property of exchanging the cations of ion-forming impurities for sodium or ammonium aluminum ions. Absorbed are in particular polyvalent cations (hardening constituents, heavy metals etc.) but also cationic tensides, quaternary organic ammonium compounds etc. Other fields of application include dust removal, water technology, in particular in water pipelines, and the use of the filters as air, coffee, smoke or dust filters.
From the treated cellulose fibers it is possible to produce in the manner known per se filter paper or paper-type nonwoven material either made exclusively of cellulose fibers or in mixture with other suitable starting materials for such filters such as, for example plastics or glass fibers.
It is then possible to produce from the filter paper or the paper-like nonwoven material different types of filters such as, for example filter bags or cartridge filters. The filters are used as one-way filters, as a rule. Filters exclusively consisting of cellulose and/or the modified cellulose-containing material as defined by the invention offer the advantage that they are completely biodegradable.
All fibers with a high cellulose content that are suitable for the manufacture of paper such as, for example cotton linters, sulfate and sulfite celluloses from various timbers, and fibers recycled from old paper can be used as cellulose fibers. The following possibilities are available with respect to the phosphorylation and carbamidation reaction:
Treatment of the entire cellulose-containing starting material prior to the manufacture of the filter paper or paper-containing nonwoven material;
treatment of a partial amount of the required cellulose starting material prior to the manufacture of the filter paper or paper-like nonwoven material, and subsequent mixing of said partial amount with untreated cellulose-containing fiber material; and
manufacture of the filter paper or paper-like nonwoven material in the manner known per se, and subsequently treatment of the web of filter paper or paper-like nonwoven material by phosphorylation and carbamidation.
Different degrees of phosphorylation and carbamidation can be adjusted for the nitrogen and phosphorus contents within the specified range limits depending on the purpose of application of the filters.
The phosphorylation and carbamidation of the cellulose-containing starting material for the production of the filter paper or the paper-type nonwoven material is carried out under the following conditions:
It is important that the cellulose-containing fiber material is brought into a particularly reactive form prior to the phosphorylation and carbamidation reaction. Such a so-called activation is carried out by adjusting the moisture content of the cellulose-containing material by adding water to it in an amount of at least 30% by mass of the cellulose-containing material. The cellulose-containing starting material usually already has a water content of from 5 to 25%. In order to achieve the desired activation it is necessary that the cellulose-containing fiber material is subjected to the action of water over a longer period of time. The duration is substantially dependent upon the already existing moisture content of the material and amounts to at lest half an hour.
The reaction partners phosphoric acid or ammonium phosphate and urea have to be admixed to the cellulose-containing material in such a way that said reaction partners are present in the material with uniform distribution after the mixing process has been completed. In addition to the aforementioned activation, attention has to be paid in particular to a uniform distribution of the reaction partners in the cellulose-containing fiber material.
It is not absolutely necessary to maintain a defined sequence for adding the reaction partners.
The activation can be advantageously combined with the mixing of urea and/or phosphoric acid or ammonium phosphate. From the amounts of urea and/or phosphoric acid or ammonium phosphate and the amount of water required for the activation, a clear solution of said components is formed, if necessary under heating of up to 60xc2x0 C. Said solution is used instead of water for activating the cellulose-containing fibrous material. In the course of the activation it is necessary only to make sure that no loss of water occurs.
An important step of the method consists in that prior to the actual phosphorylation and carbamidation, the moisture present in the reaction mixture for the purpose of activation is almost completely expelled. This is achieved by heating the mixture to temperatures of from 60xc2x0 to 100xc2x0 C. while applying a vacuum at the same time. Only once the water has been distilled off is it permissible to start the phosphorylation and carbamidation reaction, which is carried out by heating the mixture to a temperature of 125 to 155xc2x0 C. while simultaneously applying a vacuum and maintaining a reaction time of at least 15 minutes.
Carrying out said reaction under vacuum leads to a number of decisive advantages of great importance is that the reaction temperature can be reduced by about 40xc2x0 C. as compared to when it is carried out under normal pressure. Secondary reactions of phosphoric acid or ammonium phosphate and urea are distinctly reduced in this way, and decomposition reactions of the cellulose-containing fibrous material are suppressed. For example, it is possible, furthermore, to reduce the amounts of the reaction components urea and phosphoric acid or ammonium phosphate used. Furthermore, a careful treatment of the cellulose-containing material is assured as the phosphorylation and carbamidation is being carried out owing to the low reaction temperatures and reduced amounts of phosphoric acid or ammonium phosphate and urea used. In this way, the structures and mechanical properties of the cellulose-containing fibrous materials are preserved in the course of the reaction to a large extent, which is very important for the manufacture of the paper or nonwoven material.
Furthermore, it is important to maintain reaction times of at least 15 minutes. If the reaction times are shorter, the phosphoric acid used, for example, is reacted incompletely, and in particular the nitrogen content will be too low. Furthermore, it has been found that after excessively long reaction times, i.e. in excess of four hours, the absorption capacity clearly diminishes, whereby the known condensation reactions among the phosphate group to diphosphates etc. obviously take place. Upon expiration of the reaction time, the reaction product is cooled to normal temperature in the manner known per se, and the impurities are washed out.
The phosphoric acid or ammonium phosphate is preferably added to the activated cellulose-containing material first and uniformly distributed, and the urea subsequently. The mixing times for admixing the phosphoric acid or ammonium phosphate and the urea amount to 15 minutes each. The reaction components phosphoric acid or ammonium phosphate and urea can be mixed with the cellulose-containing material also at room temperature. Prior to the activation, the cellulose-containing material can be heated to the temperature of the solution of urea and/or phosphoric acid or ammonium phosphate in water.
Any desired technical quality can be used as phosphoric acid, in particular the commercially available 85% grade. Furthermore, instead of the total or part of the phosphoric acid it is possible also to use equivalent amounts of the ammonium phosphates. Urea is preferably suited in the pelletized form; however, any other technical, commercially available urea is suitable as well.
According to the proposed method, even only small amounts of urea and phosphoric acid or ammonium phosphate lead to fibers with high absorptive capacity. This applies to both the absorptive capacity and the strength with which in particular heavy metals are bound.
The phosphorylation and carbamidation of filter paper or paper-like nonwoven material that has already been produced previously in the form of webs from cellulose-containing material is carried out under the following conditions: said starting material is treated with a solution of phosphoric acid and/or ammonium phosphate and urea in water at a molar ratio of urea to phosphorus of 2.5:1 to 4.5:1, whereby the amount of water is adjusted in such a way that 1 to 8 mols phosphorus per kg cellulose remain in the cellulose-containing starting material. The starting material can be treated on one or both sides by coating it with the solution, or it is impregnated in a bath of the solution in a device operating in cycles.
The water is completely expelled by a subsequent vacuum treatment with simultaneous heating of the starting material to a temperature of 60xc2x0 to 100xc2x0 C. Thereafter, the phosphorylation and carbamidation reaction is carried out under vacuum as well, at a temperature of 125xc2x0 to 155xc2x0 C. and in the course of a reaction time of at least 15 minutes. The vacuum is preferably adjusted in each case to a value of 5.33 kPa to 26.66 kPa.
In connection with filters used in applications for drinking water, the present ammonium form is converted before the phosphorylated and carbamided cellulose-containing material is washed and dried into the sodium form by treating it with a solution of common salt. The treatment is carried out either on modified fibers prior to the actual manufacture of the paper, or on the modified filter paper or nonwoven material.